A hydraulic mechanism, engine or pump, is already known, for example by French Pat. No. 2 160 120 (FIGS. 4, 5 and 6), which comprises a cam, a cylinder block mounted to rotate with respect to the cam, a plurality of cylinders arranged in this cylinder block, a plurality of pistons each mounted to slide inside a cylinder along an axis of slide (of the piston), a plurality of abutment rollers for rolling of the pistons on the cam, each roller being mounted on a piston. Each roller is rotatable about a roller axis perpendicular to the axis of slide of said piston by means of a rotation bearing which is axially defined by two substantially plane and parallel, transverse end faces and is capable of penetrating, at least partially, inside the cylinder in which this piston is mounted to slide. The end of said piston, in which is arranged said rotation bearing of the roller, further comprises two recesses which are defined by two substantially plane, parallel faces, and means for holding the rollers in position in their respective bearings, rendered fast with the cylinder block and preventing any escape of each roller from its bearing by sliding in the direction of the roller axis of said roller. The cylinder block comprises at least one row of cylinders whose axes are substantially contained in the same transverse plane perpendicular to the axis of rotation of the cylinder block with respect to the cam and are disposed radially, the means for holding any two rollers in position, corresponding to two successive cylinders belonging to the same row of cylinders, are constituted by pieces distinct from one another.
According to this known embodiment, a sort of cap is positioned in the upper part of each cylinder and possesses a central opening to allow passage of the roller located in the upper part of the piston mounted in the cylinder. This arrangement is not suitable for the embodiments incorporating two rows of cylinders, disposed side by side nor, in general, for the numerous variants which may be made, such as those in which the roller is not disposed symmetrically with respect to the plane containing the axes of the various cylinders of a row.
According to another known embodiment, likewise disclosed by French Pat. No. 2 160 120 (FIGS. 1 and 2), just for the holding means fast with the cylinder block, it is known to produce rings adapted to be disposed in grooves provided in the cylinder block, on each side of each row of cylinders and fixed thereto. The corresponding machining is considerable and expensive, and the positioning sometimes delicate.
The known means for holding the rollers in position are either not very easy to use or present certain difficulties with respect to the application thereof.